OC基础学习2：面向对象编程的基础知识

间接(indirection)

“只要再多添加一层间接，计算机科学中就没有解决不了的问题。”

例子
- 电话薄
- 让他人代替你自己去完成工作
- 编写一段代码来查询其他代码，并通过它继续访问另一层代码。
- 推诿
变量与间接
使用文件名的间接

在面向对象编程中使用间接

使用间接来调用代码，不是直接调用某个函数，而是间接调用。

过程式编程(Procedual Programming)

#import <Foundation/Foundation.h>

typedef enum {
    kCircle,
    kRectangle,
    kEgg,
} ShapeType;

typedef enum {
    kRedColor,
    kGreenColor,
    kBlueColor
} ShapeColor;

//不同图形元素
typedef struct {
    int x, y, width, height;
} ShapeRect;

//图形结构
typedef struct {
    ShapeType type;
    ShapeColor fillColor;
    ShapeRect bounds;
} Shape;

//颜色函数
NSString *colorName(ShapeColor fillColor)
{
    switch (fillColor) {
        case kRedColor:
            return @"red";
            break;
        case kGreenColor:
            return @"green";
            break;
        case kBlueColor:
            return @"blue";
            break;

    }
    return @"no clue";
}

//绘制图形
void drawCircle(ShapeRect bounds, ShapeColor fillColor)
{
    NSLog(@"drawing a Circle at (%d %d %d %d) in %@", bounds.x, bounds.y, bounds.width, bounds.height, colorName(fillColor));
}
void drawRectangle(ShapeRect bounds, ShapeColor fillColor)
{
    NSLog(@"drawing a Rectangle at (%d %d %d %d) in %@", bounds.x, bounds.y, bounds.width, bounds.height, colorName(fillColor));
}
void drawEgg(ShapeRect bounds, ShapeColor fillColor)
{
    NSLog(@"drawing a Egg at (%d %d %d %d) in %@", bounds.x, bounds.y, bounds.width, bounds.height, colorName(fillColor));
}
void drawShapes(Shape shapes[], int count)
{
    for (int i=0; i<count; i++) {
        switch (shapes[i].type) {
            case kCircle:
                drawCircle(shapes[i].bounds, shapes[i].fillColor);
                break;
            case kRectangle:
                drawRectangle(shapes[i].bounds, shapes[i].fillColor);
                break;
            case kEgg:
                drawEgg(shapes[i].bounds, shapes[i].fillColor);
                break;
            default:
                break;
        }
    }
}

int main(int argc, const char * argv[]) {
    Shape shapes[3];
    
    ShapeRect rect0 = {0, 0, 10, 30};
    shapes[0].type = kCircle;
    shapes[0].fillColor = kRedColor;
    shapes[0].bounds = rect0;
    
    ShapeRect rect1 = {30, 40, 50, 60};
    shapes[1].type = kRectangle;
    shapes[1].fillColor = kGreenColor;
    shapes[1].bounds = rect1;
    
    ShapeRect rect2 = {15, 18, 37, 29};
    shapes[2].type = kEgg;
    shapes[2].fillColor = kBlueColor;
    shapes[2].bounds = rect2;
    
    drawShapes(shapes, 3);
    
    return 0;
}

修改过去正常工作的代码很可能会引入新的错误。 建立在函数之上，数据为函数服务。 代码例子 3.2.1 Shapes-Procedural

面向对象编程
- 以数据为中心，函数为数据服务
- 代码例子 3.2.2 Shapes-Object
- id是一种泛型，可以用来引用任何类型的对象(id实际上是一个指向结构体的指针)。
- 方括号在OC中其他意义：用于通知某个对象该去做什么。[shape draw];表示通知shape对象执行draw操作
- 发送消息（调用方法）：通知对象执行某种操作。
- 类是一种能够实例化成对象的结构体。
- 如果在运行时改变某个类，则该类的所有对象自动继承这些变化。

#import <Foundation/Foundation.h>

typedef enum {
    kRedColor,
    kGreenColor,
    kBlueColor
} ShapeColor;

//不同图形元素
typedef struct {
    int x, y, width, height;
} ShapeRect;

//颜色函数
NSString *colorName(ShapeColor fillColor)
{
    switch (fillColor) {
        case kRedColor:
            return @"red";
            break;
        case kGreenColor:
            return @"green";
            break;
        case kBlueColor:
            return @"blue";
            break;
            
    }
    return @"no clue";
}


@interface Circle : NSObject
{
    @private
    ShapeColor fillColor;
    ShapeRect bounds;
}
- (void) setFillColor: (ShapeColor) fillColor;
- (void) setBounds: (ShapeRect) bounds;
- (void) draw;

@end  //Circle
@implementation Circle
- (void) setFillColor:(ShapeColor) c
{
    fillColor = c;
}

- (void) setBounds:(ShapeRect) b
{
    bounds = b;
}

- (void) draw
{
    NSLog(@"drawing a Circle at (%d %d %d %d) in %@", bounds.x, bounds.y, bounds.width, bounds.height, colorName(fillColor));
}
@end


@interface Rectangle : NSObject
{
@private
    ShapeColor fillColor;
    ShapeRect bounds;
}
- (void) setFillColor: (ShapeColor) fillColor;
- (void) setBounds: (ShapeRect) bounds;
- (void) draw;

@end  //Rectangle
@implementation Rectangle
- (void) setFillColor:(ShapeColor) c
{
    fillColor = c;
}

- (void) setBounds:(ShapeRect) b
{
    bounds = b;
}

- (void) draw
{
    NSLog(@"drawing a Rectangle at (%d %d %d %d) in %@", bounds.x, bounds.y, bounds.width, bounds.height, colorName(fillColor));
}
@end


@interface Egg : NSObject
{
@private
    ShapeColor fillColor;
    ShapeRect bounds;
}
- (void) setFillColor: (ShapeColor) fillColor;
- (void) setBounds: (ShapeRect) bounds;
- (void) draw;

@end  //Egg
@implementation Egg
- (void) setFillColor:(ShapeColor) c
{
    fillColor = c;
}

- (void) setBounds:(ShapeRect) b
{
    bounds = b;
}

- (void) draw
{
    NSLog(@"drawing a Egg at (%d %d %d %d) in %@", bounds.x, bounds.y, bounds.width, bounds.height, colorName(fillColor));
}
@end


// 补充一个三角形
@interface Triangle : NSObject
{
@private
    ShapeColor fillColor;
    ShapeRect bounds;
}
- (void) setFillColor: (ShapeColor) fillColor;
- (void) setBounds: (ShapeRect) bounds;
- (void) draw;

@end  //Triangle
@implementation Triangle
- (void) setFillColor:(ShapeColor) c
{
    fillColor = c;
}

- (void) setBounds:(ShapeRect) b
{
    bounds = b;
}

- (void) draw
{
    NSLog(@"drawing a Triangle at (%d %d %d %d) in %@", bounds.x, bounds.y, bounds.width, bounds.height, colorName(fillColor));
}
@end

void drawShapes(id shapes[], int count)
{
    for (int i=0; i<count; i++) {
        id shape = shapes[i];
        [shape draw];
    }
}

int main(int argc, const char * argv[]) {
    
    id shapes[4];
    
    ShapeRect rect0 = {0, 0, 10, 30};
    shapes[0] = [Circle new];
    [shapes[0] setBounds:rect0];
    [shapes[0] setFillColor:kRedColor];
    
    ShapeRect rect1 = {30, 40, 50, 60};
    shapes[1] = [Rectangle new];
    [shapes[1] setBounds:rect1];
    [shapes[1] setFillColor:kGreenColor];

    ShapeRect rect2 = {15, 18, 37, 29};
    shapes[2] = [Egg new];
    [shapes[2] setBounds:rect2];
    [shapes[2] setFillColor:kBlueColor];
    
    ShapeRect rect3 = {3, 4, 5, 0};
    shapes[3] = [Triangle new];
    [shapes[3] setBounds:rect3];
    [shapes[3] setFillColor:kBlueColor];
    
    drawShapes(shapes, 4);
    
    return 0;
}

有关术语

class
object
instance
message
method
method dispatcher
interface
implementation

OC中的OOP

@interface Circle : NSObject
{
    @private
    ShapeColor fillColor;
    ShapeRect bounds;
}
- (void) setFillColor: (ShapeColor) fillColor;
- (void) setBounds: (ShapeRect) bounds;
- (void) draw;

@end  //Circle
@implementation Circle
- (void) setFillColor:(ShapeColor) c
{
    fillColor = c;
}

- (void) setBounds:(ShapeRect) b
{
    bounds = b;
}

- (void) draw
{
    NSLog(@"drawing a Circle at (%d %d %d %d) in %@", bounds.x, bounds.y, bounds.width, bounds.height, colorName(fillColor));
}
@end

@interface OC编译器需要一些有关类的信息
- @ 可以看成是对C语言的扩展
- instance variable(实例变量) ： @interface下的花括号的内容：

{ @private ShapeColor fillColor; ShapeRect bounds; } + **method declaration**(方法声明)。有点像C语言中的函数原型。`-`表示对象方法，`+`表示类方法。`(void)`表示返回类型。 - (void) setFillColor: (ShapeColor) fillColor; - (void) setBounds: (ShapeRect) bounds; - (void) draw; ```

+ **infix notation**(中缀符) ： *方法的名称及其参数都是合在一起的*   
    `[circle setFillColor: kRedColor]` 表示调用带一个参数的方法
+ 如果方法使用参数，则需要冒号，否则不需要冒号
+ 提倡@end语言后添加注释来注明类的名称

@implementation
- @implementation中可以定义在@interface中声明过和没有声明过的方法
- OC中不存在真正的私有方法
实例化对象
- instantiation（实例化）
- [Circle new] 发送new消息
软件实体应该对扩展开放，而对修改关闭。 —- 开放/关闭原则(Bertrand Meyer)